1. Field of the Invention
The present invention relates to a magnetic switch for use mainly in a coaxial type starter to start an engine.
2. Description of Related Art
As a conventional coaxial type starter device for starting an engine, one structured as disclosed in Japanese Patent Application Laid Open No. 63-140864 (1988) has been well known. FIGS. 1 and 2 show sectional views of a magnetic switch employed in the conventional coaxial type starter device. FIG. 1 represents a case where a plunger 7 which will be described later is at a stationary or rest position, while FIG. 2 represents a case where the plunger is at a fully shifted position.
In FIGS. 1 and 2, a reference numeral 1 designates a rear end portion of a direct current motor which produces starting torque for an engine. A magnetic switch 2 is provided at the rear end side of the motor 1, which is coupled to an armature shaft (not shown) of the motor 1. The armature shaft is coupled at the front end side of the motor 1 to an output shaft transmitting the rotation of the motor 1 to the engine. The magnetic switch 2 not only slides the output shaft, but allows power supply from a battery to the motor 1 when a key switch of a vehicle is turned on.
The magnetic switch 2 has an iron casing 3 outside, in which an annular front core 4a and an annular rear core 4b form a magnetic path together with the casing 3, at the front end in contact with the motor 1 and at the rear end thereof respectively. An inner periphery of the rear core 4b projects towards the front core 4a thereby to form a cylindrical portion 4c. The length of the cylindrical portion 4c is equal to the length in the axial direction of an outer peripheral surface of the plunger 7 described later.
Between the front and rear cores 4a and 4b is provided a plastic hollow bobbin 5 wound with an exciting coil 6. The bobbin 5 insulates the exciting coil 6 from the front and rear cores 4a and 4b. A cylindrical plunger 7 is slidably arranged in the hollow at the center of the bobbin 5. A plunger restoring spring 17 is provided between the front end of the plunger 7 and the front core 4a so as to restore the plunger 7 from the fully shifted position to the stationary position. When the plunger 7 is in the stationary position as shown in FIG. 1, the peripheral surface of the plunger 7 confronts the whole peripheral surface of the cylindrical portion 4c of the rear core 4b. When the plunger 7 is fully shifted frontward as shown in FIG. 2, merely a slight peripheral portion at the rear end of the plunger 7 confronts the front end of the peripheral surface of the cylindrical portion 4c.
The inner periphery at the rear end of the plunger 7 is integrally formed with an intermediate plate 7a, in the center of which is coupled one end of a plunger rod 8. Another end of the plunger rod 8 enters the hollow armature shaft from the rear end of the motor 1 to be coupled with a middle rod (not shown) inside the hollow of the armature shaft. The plunger rod 8 transmits the shifting force of the plunger 7 to the output shaft via the middle rod. Around the plunger rod 8 at the coupled side to the intermediate plate 7a is fitted a sleeve 9. A traveling contact point 11 is so held around the sleeve 9 having an insulating body 10 therebetween as to be slidable in the axial direction of the plunger rod 8.
In the magnetic switch 2 having the aforementioned structure, when the exciting coil 6 is supplied power, the plunger 7 is attracted by the front core 4a to be shifted forward because of the magnetic attraction force by magnetic flux through the casing 3, rear core 4b, plunger 7 and front core 4a. When the plunger 7 is attracted frontward, the plunger rod 8 slides the output shaft frontward, whereby the output shaft is projected outside the coaxial type starter device. Consequently, the traveling contact point 11 held on the plunger rod 8 is brought into touch with a fixed contact point 12 provided at a predetermined position, so that a power supply circuit from the battery to the motor 1 is formed.
FIG. 3 is a graph showing the relation between the magnetic attraction force and, a gap (g) between the front end of the plunger 7 and the rear end of the front core 4a (plunger gap). In the graph, a curve A indicates the relation between them in the conventional magnetic switch 2 having the above-described structure, wherein as the plunger gap (g) is narrower, the rate of increase in the attraction force lowers. However, because a larger restoring force is added to the plunger 7 due to the compression of the plunger restoring spring 17 as the plunger gap (g) gets narrower, the plunger 7 cannot be attracted up to the front core 4a unless the magnetic attraction force exceeds the restoring force.
Theoretically, as the plunger 7 comes nearer to the fully shifted position to narrow the plunger gap (g), an increasing rate of the magnetic attraction force the plunger 7 receives from the front core 4a becomes higher. In practice, on the contrary, an increasing rate of the magnetic attraction force lowers as the plunger gap (g) becomes narrower. The reason for this is considered as follows. As the plunger 7 is shifted frontward thereby to reduce the area of the cylindrical portion 4c of the rear core 4b confronting the plunger 7, magnetic-flux 13 as indicated in FIG. 2 is generated between the cylindrical portion 4c and the rear end of the plunger 7. This magnetic flux 13 acts as a reverse attraction force to pull back the plunger 7 rearward, and therefore the magnetic attraction force in a forward direction is reduced by the reverse attraction force. Without sufficient attraction force for the plunger 7, when the source voltage lowers, for instance, a predetermined attraction force cannot be obtained.
As the first method for preventing the generation of the magnetic flux 13, it may be considered to lengthen the plunger to confront the peripheral surface thereof with the whole peripheral surface of the cylindrical portion 4c even when the plunger is fully shifted. However, to lengthen the plunger increases the total length of the magnetic switch 2, which conflicts with the technology trend toward a compact but highly efficient device.
As the second method, as shown in FIG. 4, it may be considered to shorten the bobbin 5 wound with the exciting coil 6 in the axial direction to remove the cylindrical portion 4c. In this case, the rear core 4b is so positioned that when the plunger is at the stationary position, the inner peripheral surface of the rear core 4b faces the front end of the peripheral surface of the plunger 7, whereas, when the plunger 7 is at the fully shifted position, the rear core 4b confronts the rear end of the peripheral surface of the plunger 7. In this second method, no magnetic flux as in the first method is generated, however a sufficient winding number cannot be secured for the exciting coil 6. Therefore, as indicated by a curve B in FIG. 3, although an increasing rate of the magnetic attraction force becomes higher as the plunger gap (g) becomes narrower, the initial attraction force is so small to obtain sufficient attraction force.